Transcript OSPF Header - Electrical & Computer Engineering

OSPF Header
OSPF HEADER
Version (1 byte) Pkt type (1 byte)
OSPF HEADER for this project
2
Type
Length of Pkt (2 bytes)
don’t care
Source Router ID (4 bytes)
IP:PORT (8 bytes!!)
Area ID (4 bytes)
don’t care
Checksum (2 bytes)
don’t care
Authentication type (2 bytes)
don’t care
Authentication data (8 bytes)
Don’t care
Types we will use
Type = 1 -> HELLO
Type = 3 -> link state request
Type = 4 -> link state update
Type = 5 -> link state ACK
HELLO Packet
standard
OSPF Header with
Type = 1 (HELLO)
OSPF Header with
Type = 1 (HELLO)
Network Mask (4 bytes)
Don’t care (4 bytes)
Hello Interval (2 bytes)
10 (2 bytes)
options
priority
this project
Don’t
care
Dead interval (4 bytes)
40 (4 bytes)
Designated router
(4 bytes)
Don’t care
(4 bytes)
Backup designated router
(4 bytes)
Don’t care
(4 bytes)
1st recent neighbor ID
(4 bytes)
1st Neighbor ID
(8 bytes)
last recent neighbor ID
(4 bytes)
SENDER ID
(8 bytes)
Neighbors are the
Ids (IP:Port) that
have been send a
HELLO since the
in the previous 40
seconds (dead
interval).
Put the senders ID
last as a delimiter.
HELLO Protocol
Every hello interval (10 sec) you must send out a hello to ALL neighbors.
The HELLOs are even sent across links that seem to be down.
During initialization, a HELLO is sent to all neighbors.
If a HELLO is received and 1. The receiving node is listed in the recent
neighbors list and 2. the sending node’s BIDIRECTIONAL flag is zero, then
set it to one and send immediately respond with a HELLO, with, of course,
the node that just sent the HELLO on the recent neighbors list.
If a HELLO is received and the receiving node is not listed in the recent
neighbors list, then immediately respond with a HELLO, with, of course, the
node that just sent the HELLO on the recent neighbors list.
Whenever a HELLO is received, set the sending node’s
LAST_HELLO_TIME to the current time.
The Link State Update packet
LSA
OSPF Header with
Type = 4 (update)
Number of LSAs
LSA1
LSA Header
LSA2
Link 1
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
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




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

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

LSA Age
options
LS type
Link State ID
4 bytes
Advertising router
4 bytes
LS Sequence Number
4 bytes
checksum
length
router type
0
Number of Links
Link ID
4 bytes
Link Data
6 bytes
Link type
metric
TOS
The Link State Update Packet for Project
LSA
OSPF Header with
Type = 4 (update)
Number of LSAs
LSA Header
LSA1
LSA2
Link 1

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


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



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



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


LSA Age
Don’t
care
Link State ID
8 bytes
don’t care
4 bytes
LS Sequence Number
4 bytes
don’t care
don’t care
Don’t
care
Number of Links
Link ID
8 bytes
Link Data
6 bytes
Link type
metric
TOS
Database Exchange
This is not according to the OSPF standard.
It will generate more control traffic than the standard, and will not work in all
situations. But, it is simpler to program and will suffice for this project.
When a link switches to bidirectional, a full database exchange should take place.
Suppose that node A has switched its bidirectional flag to node B to one,
then node A initializes a database exchange:
•Node A sends an OSPF link state request packet to node B.
•Node B responds by all its LSA’s in one packet.
•If A does not get a LSA from B within 2 seconds, A resends an OSPF link state request packet to node B until either it
gets an LSA or the link is no longer BIDIRECTIONAL.
•When A receives the LSA database, it responds with a link state ACK packet.
•If B fails to get a link state ACK within 2 seconds, B retransmits the LSA database. (Remember to not miss sending and
receiving HELLO messages while sending a database).
•B continues to retransmit the LSA database until the link is no longer BIDIRECTIONAL or the entire database has been
sent.
Note, since both ends will switch the link to bidirectional, both ends will initiate a database exchange
Reception of link state update
A link state update is made up of one or more LSAs
When an link state update arrives, send a link state ACK to sending node (not quite the standard).
•For each in the LSA in the update
•if the sequence number is larger than the sequence number in the current
database then
•If the Link State ID of in the LSA is not the same as this node’s ID, then
•Update link state database
•Flood this LSA to all neighbor except the one that sent it.
•else
•Generate a new LSA with sequence number equal to one plus the
sequence number in the arrived LSA.
•Flood this LSA to ALL neighbors.
•If the LSA sequence number is less than or equal to the one currently in the
database, then don’t do anything.
Reliable Flooding
• When a packet is to be flooded, put it in the
ToBeFlooded list.
• This list should contain the packet to be flooded,
the address to where it should be sent and the time
out, the next time it should be resent.
• Hint: Since you should check periodically if
packets need to be resent, you can initially put the
packet in the list with an expired time out.
• When you get an ACK, the packet should be
removed from the ToBeFlooded list.
Initialize
Send HELLOs
wait for packet arrival
(but don’t wait more than 2 sec)
receive LS update
receive HELLO
Forward data packet
Reply to HELLO ?
receive LS ACK
Clear entry on ToBeFlooded List
receive LS request
Reply to HELLO
Possibly clear
WaitingForUpdate
flag
does update
contains new LSAs?
Set Bidirectional?
update LSA database
Set
compute new routing table flag
Set Bidirectional
and
Request LS update
Set waiting for LS update from XXX flag0
Add to ToBeFlooded List
Any LS update request
need to be resent?
Send HELLOs if needed
Make new routing table
If needed
send ToBeFlooded list entries
Any overdue HELLOs?
Clear LSA with MAXAGE
and add ToBeFlooded list
yes
Age LSA
Any overdue LSA?
yes
Delete link
Set compute new routing
table flag
Generate new LSA
Add to be flooded list